Voltage controlled current driver powered by negative voltage rail

ABSTRACT

A method and current drive circuit is provided that accepts a positive voltage input signal and supplies power to a load from a negative voltage rail.

BACKGROUND

A torque motor is an inductive device that is controlled with a currentbased driver circuit. Torque motors can be used in any number ofapplications using pneumatic devices that need controllable flow.Examples of devices which employ torque motors are bleed systems,anti-icing systems, engine control systems, environmental controlsystems, and air management systems. The torque motor responds to thedifference in pressure between the supply and demand sides to provide aregulated air flow. It provides more flow when more current is suppliedto it. In this way, the flow is related to the current and can beregulated by using a driver with an adjustable current output.

Torque motor drivers are generally one of two types: linear voltagecontrolled current drivers and pulse width modulation (PWM) drivers. PWMdrivers can use both a positive and negative voltage rail, but they havegreater electromagnetic noise interference problems, require moresophisticated control techniques, and use more parts and layout space.Linear voltage controlled devices are simpler to implement and havebetter noise characteristics, which make them a preferred method in manyimplementations of a torque motor driver. A 50 mA current driver maysupply 0 mA to the load at a command in put voltage of 0V, 25 mA at 2.5V, and 50 mA at 5V, etc.

Known linear voltage controlled current drivers are driven by a positivevoltage signal and powered by a positive voltage rail. Typical valuesare 0-5V input signals and 15-19 volt rails.

SUMMARY

A current drive circuit has an input terminal that receives a positivevoltage control signal. A power input connects to a negative voltagepower supply. Circuitry is configured to supply a controller current toan attached load from the negative voltage power supply. The magnitudeof the controlled current is based on the positive voltage controlsignal.

Another embodiment of the invention is a method for supplying current toa load. A positive voltage input signal is received from a controller.Current is supplied to the load from a negative voltage power supply.The magnitude of the current is based on the magnitude of the positivevoltage input signal.

An alternate embodiment of the present invention is a system forsupplying power to loads. A negative supply current drive circuit isconfigured to supply current to an inductive load from a negativevoltage power supply. The magnitude of the current is selected based onthe magnitude of a positive voltage input signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a system diagram illustrating current drivers and loadspowered by a positive voltage rail as known in the prior art.

FIG. 1B is a circuit diagram illustrating a current driver powered by apositive voltage rail as known in the prior art.

FIG. 2A is a system diagram illustrating current drivers and loadspowered by a positive and negative voltage rail according to anembodiment of the present invention.

FIG. 2B is a circuit diagram illustrating a current driver powered by anegative voltage rail according to an embodiment of the presentinvention.

DETAILED DESCRIPTION

Driving a load from the negative rail offers the advantage of balancingthe power drawn from the positive and negative rails. Known methods ofshifting the load to the negative voltage rail require changing thereference of the whole circuit lower towards the negative voltage rail.The problem, however, is that the voltage input signal would have toshift as well. This would require an alteration of the control circuitryor the addition of a level shifter to accommodate the change. Anadvantage of the present invention is that there is no need to changethe control circuitry providing the voltage input signal. It can remaina positive 0-5V signal and the load is appropriately supplied from thenegative voltage rail instead of the positive voltage rail. Othermethods of driving a load from the negative voltage rail do not havethis advantage.

FIG. 1A is a system diagram illustrating current drivers and loadspowered by a positive voltage rail. System 10 includes current drivers12 a-12 c powered from positive voltage rail 14. Loads 16 a-16 c arepowered by current drivers 12 a-12 c respectively. Controller 18 ispowered by positive voltage rail 14 and supplies positive voltagesignals to current drivers 12 a-12 c. The positive voltage signalscontrol the magnitude of the current supplied to loads 16 a-16 c bycurrent drivers 12 a-12 c. Negative voltage rail 20 is available butunused by current drivers 12 a-12 c. This causes an imbalance betweenthe power supplied by positive voltage rail 14 and negative voltage rail20.

FIG. 1B is an exemplary circuit that may be employed by current drivers12 a-12 c (from FIG. 1A) to power a load from a positive voltage rail.Circuit 40 has voltage input signal 42 connected to resistor R1.Resistor R1 is connected to inverting input 44 of operational amplifier(op amp) 46. Op amp 46 is powered by positive supply 48 and negativesupply 50. In this example, positive supply 48 is connected to positivevoltage rail contact 22 and negative supply 50 is connected to groundcontact 54. Output 55 is connected to negative feedback capacitor 56.Negative feedback capacitor 56 is connected back to inverting input 44and provides transient suppression. Output 56 is also connected toresistor R2. Resistor R2 is connected to the gate of p-channel fieldeffect transistor (FET) 58 and resistor R3 which is connected to thesource of p-channel FET 58 and positive voltage rail contact 60. Thedrain of p-channel FET 28 is connected to resistors R4 and R5. ResistorR4 is connected to non-inverting input 62 of op amp 46 and resistor R6which is connected to ground contact 64. Resistor R5 is connected todiode 65 and resistor R7 which is connected to inverting input 44 op amp46. Load 66 is connected to diode 65. Load 66 is the torque motor orother current based load. Diode 65 is provided only for protection andis not required.

Resistors R2 and R3 are used to properly bias p-channel FET 58. Atypical value for both resistors is 4.99 k ohms. Resistor pairs R1/R7and R6/R4 are used as voltage dividers and serve to configure op amp 16as a feedback amplifier. Typical values are 200 k ohms for R1 and R6 and20 k ohms for R7 and R4 to provide a gain of 0.1 from input voltagesignal 42 to the voltage drop on resistor R5. Resistor R5 is used toadjust the gain of the current driver. A typical value is 10 ohmsproviding a 50 mA output for a 5V input signal. With this configuration,circuit 40 is a linear voltage controlled current driver driven off thepositive voltage rail controlled by a positive input voltage.

Some torque motor applications have a positive and negative voltage railavailable. When there are multiple torque motors which in turn requiremultiple drivers, the loading in the positive and negative voltage railsbecomes imbalanced due to the current drivers all drawing from thepositive voltage rail. The present invention addresses this by providinga current driver powered by the negative voltage rail. By powering sometorque motors from the positive voltage rail and some from the negativevoltage rail, the loads can be balanced between the two rails. Currentdrivers such as the one shown in FIG. 1B can not be operated at anegative voltage and are therefore incapable of operating off thenegative voltage rail.

FIG. 2A is a system diagram of an embodiment of the present inventionillustrating current drivers and loads powered by a positive andnegative voltage rail. System 100 includes current drivers 102 a-102 cand 104 a-104 c. Current drivers 102 a-102 c are powered from positivevoltage rail 106 and current drivers 104 a-104 c are powered fromnegative voltage rail 108. Loads 110 a-f are powered by current drivers102 a-102 c and 104 a-104 c. Controller 112 is powered by positivevoltage rail 106 and supplies positive voltage signals to currentdrivers 102 a-102 c and 104 a-104 c. Current drivers 104 a-104 c acceptthe same positive voltage signal as current drivers 102 a-102 c. Fromthe perspective of controller 112 or loads 110 a-110 f, current drivers104 a-104 c are identical to current drivers 102 a-102 c. They areinterchangeable and allow the system designer to shift loads frompositive voltage rail 106 to negative voltage rail 108 to balance thepower being drawn from the two rails.

FIG. 2B is an illustration of an embodiment of the present invention fora current driver driven from the negative voltage rail. Circuit 150 hasvoltage input signal 151 connected to resistor R11. Resistor R11 isconnected to non-inverting input 152 of op amp 154. Op amp 154 ispowered by positive supply 156 and negative supply 158. In this example,positive supply 156 is connected to positive voltage rail contact 160and negative supply 158 is connected to negative voltage rail contact162. Alternatively, positive supply 156 can be connected to a groundcontact. Output 160 is connected to negative feedback capacitor 162which is connected to inverting input 164 of op amp 154. Negativefeedback capacitor 162 serves the same transient suppression purpose asnegative feedback capacitor 56 in FIG. 1B. Output 160 is also connectedto resistor R12. Resistor R12 is connected to the gate of n-channel FET166 and resistor R13 which is connected to the source of n-channel FET166 and negative voltage rail contact 168. The drain of n-channel FET166 is connected to resistors R14 and R15. Resistor R14 is connected tonon-inverting input 152 of op amp 154. Resistor R15 is connected todiode 169 and resistor R17 which is connected to inverting input 164 ofop amp 154. Load 170 is connected to diode 169. Diode 169 is providedonly for protection and is not required.

Resistors R2 and R3 are used to properly bias n-channel FET 66 and canbe 4.99 k ohms each. Resistor pairs R11/R17 and R16/R14 are used toconfigure op amp 154 as a feedback amplifier and can have values of 200k ohms for R16 and R16 and 20 k ohms for R17 and R14 to provide a gainof 0.1 from input voltage signal 151 to the voltage across resistor R15.Resistor R15 is the sense resistor placed in the current path of load170. The voltage drop on R15 in the feedback loop for op amp 154 can beused to adjust the gain of the drive circuit by changing the resistancevalue of the resistor. A nominal value for R15 is 10 ohms. This allows a0-5V input signal to provide a 0-50 mA current to load 160. The feedbackamplifier is used to control the gate voltage of n-channel FET 166 toclose the control loop and achieve the desired current through load 170.

When using the exemplary values given for resistors R11-R17 and a −15Vnegative voltage rail voltage, the output of amplifier 154 is near −15Vwhen positive input voltage 151 is at 0V. This causes n-channel FET 166to remain off. As positive input voltage 151 increases, the output ofamplifier 154 will also increase turning on n-channel FET 166 andincreasing the voltage drop on R15. At a 5V positive input voltage,amplifier 154 will increase the gate voltage of n-channel FET until itis supplying 50 mA of current to load 170. The 50 mA current creates a500 mV drop on resistor R15. Because of the 0.1 gain setup by resistorsR11, R14, R16, and R17, this balances the feedback circuit.

The positive voltage input is connected to the non-inverting input ofthe op-amp which has power supplies that accommodate a negative voltage.The second amplifier (the FET) is an n-type. This makes it possible tosupply a controllable current from a negative voltage rail using thesame positive input voltage to control the current. Using thisarchitecture, a system with available positive and negative voltagerails (ex +15V and −15V) can be balanced by driving some of the loadsfrom the negative voltage rail. Control of the circuit remainsidentical, which reduces the impact of changing the drive circuit to theremainder of the system which allows retrofitting.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

1. A current drive circuit comprising: an input terminal for receiving apositive voltage control signal; a power input for connection to anegative voltage power supply; and circuitry configured to supply acontrolled current to an attached load from the negative voltage powersupply, a magnitude of the controlled current based on the positivevoltage control signal.
 2. The current drive circuit of claim 1 whereinthe circuitry further comprises: a feedback amplifier having an outputand configured to accept the positive voltage control signal; and asecond amplifier connected to the output of the feedback amplifier andconfigured to supply the controlled current to the attached load fromthe negative voltage power supply.
 3. The current drive circuit of claim2 wherein the feedback amplifier comprises an operational amplifier. 4.The current drive circuit of claim 2 wherein the second amplifier is ann-channel field effect transistor.
 5. The current drive circuit of claim2 wherein the positive voltage control signal is in the range of 0-5 V.6. The current drive circuit of claim 1 further comprising: a groundreference; a first amplifier having an output, an inverting input and anon-inverting input; a second amplifier having an input connected to theoutput of the first amplifier and an output adapted to supply thecontrolled current; a current sense resistor connected between theoutput of the second amplifier and the attached load; a first resistorconnected between the non-inverting input of the first amplifier and theinput terminal; a second resistor connected between the non-invertinginput of the first amplifier and the output of the second amplifier; athird resistor connected between the inverting input of the firstamplifier and the ground reference; and a fourth resistor connectedbetween the inverting input of the first amplifier and the attachedload.
 7. The current drive circuit of claim 6 further comprising acapacitor connected to the inverting input of the amplifier and theoutput of the amplifier.
 8. The current drive circuit of claim 6 whereinthe positive voltage control signal is in the range of 0-5 V.
 9. Thecurrent drive circuit of claim 6 wherein the first amplifier is anoperational amplifier having a positive power supply terminal and anegative power supply terminal, where the positive supply terminal isconnected to a positive voltage power supply and the negative supplyterminal is connected to a negative voltage power supply.
 10. Thecurrent drive circuit of claim 6 wherein the first amplifier is anoperational amplifier having a positive power supply terminal and anegative power supply terminal where the positive supply terminal isconnected to a ground contact and the negative supply terminal isconnected to a negative voltage power supply.
 11. The current drivecircuit of claim 6 wherein the second amplifier is an n-channel fieldeffect transistor.
 12. The current drive circuit of claim 11 furthercomprising: a gate terminal, source terminal, and drain terminal on then-channel field effect transistor; a first biasing resistor connectedbetween the output of the first amplifier and the gate terminal of then-channel field effect transistor; and a second biasing resistorconnected between the gate terminal and the source terminal of then-channel field effect transistor; wherein the current sense resistor isconnected between the drain terminal of the n-channel field effecttransistor and the attached load.
 13. A method for supplying current toa load comprising: receiving a positive voltage input signal from acontroller; supplying current to the load from a negative voltage powersupply where a magnitude of the current is based on a magnitude of thepositive voltage input signal.
 14. The method of claim 13 whereinproviding the positive input voltage signal comprises providing a 0-5Vinput voltage signal.
 15. The method of claim 13 wherein supplyingcurrent to the load comprises supplying a current in the range ofapproximately 0-50 mA.
 16. The method of claim 13 wherein supplyingcurrent to the load comprises: comparing a voltage drop on a currentsense resistor to the positive voltage input signal using a feedbackamplifier; supplying a control signal to a second amplifier from anoutput of the feedback amplifier; supplying current to the load from thesecond amplifier based on a magnitude of the control signal.
 17. Themethod of claim 16 wherein the second amplifier is an n-channel FET. 18.A system for supplying power to inductive loads comprising: at least oneinductive load; at least one negative supply current driver circuitconfigured to supply current to the inductive load from a negativevoltage power supply where a magnitude of the current is selected basedon a magnitude of a positive voltage input signal.
 19. The system ofclaim 18 further comprising: at least a second inductive load; at leastone positive supply current driver circuit configured to supply currentto the second inductive load from a positive voltage power supply wherea magnitude of the current is selected based on a magnitude of apositive voltage input signal.
 20. The system of claim 19 furthercomprising: a plurality of loads; a plurality of negative supply currentdrive circuits; and a plurality of positive supply current drivecircuits; wherein each of the negative supply current drive circuits andthe positive supply current drive circuits is connected to at least oneof the plurality of loads such that a total power drawn from thepositive voltage power supply is approximately equal to a total powerdrawn from the negative voltage power supply.